Level transmitters or level transducers are used to measure the level of a liquid or bulk solid material (as well as slurries) within a specified space, and to provide electrical output about these measurements that are proportional to the input level.  Level transmitters are designed to measure level based on either a point level or continuous level readings.  Point level transmitters provide output when a specific level measurement is reached.  This output is generally in the form of an audible alarm or an electrical charge to actuate a switch. Multiple point level transmitters can be integrated together to give a stepped version of continuous level.  Continuous level transmitters measure level within a specified range and provide output as a continuous reading of the level.  Level transmitters can operate through a wide range of temperatures, pressures, vapor gas mixtures, and process conditions. 

Level transmitters can use many different technologies to sense or measure level.  Some of these include air bubblers, which use a constant volumetric rate of gas flow (usually air) fed through a bubble tube submerged in a liquid. The amount of pressure required to force the bubble gas out of the bottom of the bubble tube is equal to the hydrostatic pressure at that point. Capacitive or RF admittance transmitters use a radio frequency technique based on differing dielectric constants. The impedance detected by the RF changes whenever material approaches the sensing region. Differential pressure level transmitters measure head pressure in a vessel due to height of material in vessel.  Electrical conductivity or resistance devices use a low-voltage power source applied across separate electrodes. A conductive liquid contacting both probes completes the conductive circuit. Mechanical or magnetic floats have sealed reed switches in their stems with a permanent magnet installed in the float. When the float rises or falls, the switch is activated. Optical level transmitters use optic sensors and principle of optical refraction to detect the presence or absence of fluid. Typically LEDs are utilized for source of light.

Some styles of level transmitters use pressure membrane as the detector.  These devices have a pressure sensitive switch with pressure transmitted to an internal sensor via an organic, or thin metal, membrane.  Other types of level transmitters include radar or microwave driven devices that emit a microwave pulse toward the process material.  The pulse is reflected back by surface of material and picked up by receiver. Level is inferred from the time of flight. Rotation paddle level transmitters use a small electric motor to drive the paddle, which rotates freely in the absence of material. When the paddle is impeded by material, the motor senses this and typically triggers a switch.  Sonic or ultrasonic level transducers measure the length of time it takes for a reflected sound wave to return to a transducer. Transmitted time is proportional to level. Vibrating or tuning fork level transmitters use a variety of technologies (typically piezoelectric) to vibrate the probe, and then constantly monitor the presence or absence of that vibration.

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